1. Field
Exemplary embodiments of the present invention relate to a nonvolatile memory device.
2. Description of the Related Art
Nonvolatile memory devices maintain data stored therein even when power supply is cut off. As an example of the nonvolatile memory devices, a floating-gate-type nonvolatile memory device may be driven by using a floating gate positioned over a channel region of a substrate and insulated from the channel region. Here, as the amount of electric charges maintained in a conductive band of the floating gate is adjusted, a threshold voltage of a floating-gate-type nonvolatile memory is shifted. When a program operation is performed on a memory cell, electrons are stored in a conductive band of a floating gate by Fouler-Nordheim (F-N) tunneling. In this case, the threshold voltage of the memory cell is raised by the electric charges stored in the conductive band of the floating gate. Here, the characteristics of memory cells inside a nonvolatile memory device are different from each other, and the memory cells have a constant threshold voltage distribution width. Hereafter, a method for storing data in memory cells of a flash memory, for example, will be described.
A nonvolatile memory device includes a memory cell array to store data. The memory cell array includes a plurality of memory blocks. Each of the memory blocks includes a plurality of pages. Each of the pages includes a plurality of memory cells. The memory cells are divided into on cells and off cells, depending on threshold voltage distributions. The on cells are erased cells, and the off cells are programmed cells. The nonvolatile memory device performs an erase operation by the memory block and performs a read or write operation by the page.
Meanwhile, the nonvolatile memory device may store one bit data in one memory cell, or store two or more bits in one memory cell. In general, a memory cell to store one bit data is referred to as a single level cell (SLC), and a memory cell to store two or more bits is referred to as a multi-level cell (MLC). The SLC has an erase state or a program state depending on a threshold voltage, and the MLC has an erase state and a plurality of program states depending on threshold voltages.
A nonvolatile memory device having MLCs may reduce the gap between the threshold voltage distributions of the program states to secure a margin between the respective program states. In general, the nonvolatile memory device having MLCs stores 2-bit data or 3-bit data in each memory cell thereof. Here, the voltage level of a voltage in the erase state and the voltage levels of threshold voltages in some program states may be set to negative voltages. In this case, the performance and reliability of the nonvolatile memory device having MLCs may be improved.
FIG. 1 is a diagram comparatively showing a conventional threshold voltage distribution 110 and a threshold voltage distribution 120 when some of threshold voltages of the program states are set to a negative voltage. Hereafter, when some of the threshold voltages of the program states are set to a negative voltage, the threshold voltage distribution 120 is referred to as ‘negative distribution’. The voltage distributions of FIG. 1 show a threshold voltage distribution of program states of a nonvolatile memory device having MLCs to store 2-bit data.
An MLC to store 2-bit data has an erase state and three program states. As described above, an adequate margin is to be obtained between the respective program states, in order to improve the performance and reliability of a nonvolatile memory device having MLCs. At this time, the gap between threshold voltage distributions of the respective program states may be reduced to secure a margin. Alternatively, as shown in the negative distribution 120 of FIG. 1, the threshold voltage distribution of some program states among the three program states may be set to a negative voltage to increase a margin.
In the case of the existing threshold voltage distribution 110 shown in FIG. 1, only the threshold voltage distribution of an erase state 111 is set to a negative voltage, and the other threshold voltage distributions of first to third program states 112 to 114 are each set to a positive voltage. In the case of the negative distribution 120, the threshold voltage distributions of an erase state 121 and a first program state 122 are set to a negative voltage, and the threshold voltage distributions of second and third program states 123 and 124 are each set to a positive voltage. The threshold voltage distribution of a program state has an upper limit. More specifically, it is difficult to program a memory cell such that the threshold voltage thereof is set above a predetermined voltage. Therefore, when the threshold voltage distribution of the first program state 122 is set to a negative voltage as in the negative distribution 120, the second and third program states 123 and 124 may be distributed so that the interval between the two states may have an adequate margin. Such a method may be more usefully utilized in the case of a nonvolatile memory device having MLCs to store 3-bit data.
First to third dotted lines 115 to 117 indicate the levels of a plurality of verification voltages for determining whether or not MLCs are normally programmed in the existing threshold voltage distribution 110. Fourth to sixth dotted lines 125 to 127 indicate the levels of a plurality of verification voltages for determining whether or not MLCs are normally programmed in the negative distribution 120. As shown in FIG. 1, the levels of the verification voltages are to be positioned between the respective threshold voltage distributions of different program states. Here, based on whether a memory cell is turned on or off when a verification voltage is applied to a floating gate of the memory cell, it is determined whether the memory cell has been normally programmed or not.
As such, when the negative distribution 120 is used, a verification voltage is set to, for example, a negative voltage 125 in order to verify whether an MLC having the first program state 122 has been normally programmed or not.